Since the American invasion of Iraq in 2003, few Iraqis have loomed as large on the country’s political stage as the politician and wheeler-dealer Ahmad Chalabi, who died Tuesday.

Shrewd, calculating and driven, Mr. Chalabi will almost certainly be remembered within Iraq as an architect of the country’s de-Baathification policy.

That effort pushed thousands of members of Saddam Hussein’s Baath Party, many of whom were Sunni Muslims, out of government jobs and deepened resentments between Sunni and Shiite Iraqis.

Years later, those divisions have only worsened, and they opened the door for the Islamic State militant group to seize large areas of Iraq’s Sunni heartland, creating a physical partition of Iraq that mirrored its sectarian differences.

While the original decision to bar Baathists from senior government positions was an American one, driven by the goal of ensuring that Hussein’s political bloc never returned to power, it was Mr. Chalabi who became its champion and quickly seized the reins as the implementer of the new policy.

“He used it as a political weapon,” said Ryan Crocker, a former United States ambassador to Iraq, who knew Mr. Chalabi from before the invasion as well as afterward.

“I never could figure out if he had the deep anti-Baathist passion of some of the other political figures or whether this was just a tool to be used,” added Mr. Crocker, who is now the dean of the Bush School of Government and Public Service at Texas A&M University.

As promulgated by Mr. Chalabi, the de-Baathification policy drove at least 50,000 people from government jobs, and some estimates put the number as high as 100,000, according to a report by the International Center for Transitional Justice, a nonprofit organization based in New York that documents mass human rights abuses and atrocities.

In some cases, people applied successfully to be reinstated, but it was a long and bureaucratic process.
At times, the policies bordered on the absurd: like a decision to remove teachers who were senior Baath Party members, which left children without instructors until they could be replaced by non-Baath Party members.

The new teachers were less experienced and were often Shiite Muslims, deepening the resentment among Iraqi Sunnis who saw Shiites as taking all the good jobs.
Worse, thousands of those who lost their jobs were not particularly strong supporters of Hussein or of Baathist ideology.

They had signed up with the party to be in the running for jobs, and then suddenly found themselves, and their families, catastrophically penalized for it.
All of this was compounded by the simultaneous move, promoted by the United States-led Coalition Provisional Authority, to disband the standing security services, including the army.

That left hundreds of thousands more people, many of them Sunnis, out of work.
When interviewed about the problems of de-Baathification, Mr. Chalabi, who was himself Shiite, spoke about the terrible things that Hussein had done to Iraq’s Shiites and Kurds.

However, since his own family left Iraq in 1958, before the rise of the Baathists, it seemed that he spoke less from first-person experience than from a sense that the anti-Baathist position was politically expedient as he angled to become a player in Shiite politics.

However, just last year, even after the Islamic State had seized on the Sunni population’s sense of alienation to take over the city of Falluja, Mr. Chalabi argued that later sectarian policies, not the de-Baathification policy as he had envisioned it, had divided the country.

Speaking to the publication Al-Monitor in February 2014, he said: “The idea behind de-Baathification was to dismantle the Baath Party’s control over state institutions and prevent its leaders from participating in the political process.

However, this was carried out on the condition that it would not affect the overwhelming majority of Baathists.”
Later, there were much more punitive measures, and that is what sealed the problems, Mr. Chalabi said. He concluded with a bit of hindsight: “Sunni participation is the solution,” he said.

“Their participation is required. What happened in Iraq confirms that no single party can control the country’s path. No important party in Iraq should ever feel that it is marginalized.”